Vinyl acetate is an important monomer in the production of polyvinyl acetate and polyvinyl alcohol products. Vinyl acetate is conventionally prepared by contacting acetic acid and ethylene with molecular oxygen to form a crude vinyl acetate composition. The reaction is typically conducted in the presence of a suitable catalyst, which may comprise palladium, an alkali metal acetate promoter, and, optionally, a co-promoter, e.g., gold or cadmium, on a catalyst support. U.S. Pat. No. 6,696,596, for example, indicates that it is well known to manufacture vinyl acetate in a reaction in the gas phase with acetic acid and oxygen or oxygen containing gasses over fixed-bed catalysts. U.S. Pat. No. 6,040,474, as another example, describes the manufacture of acetic acid and/or vinyl acetate using two reaction zones wherein the first reaction zone comprises ethylene and/or ethane for oxidation to acetic acid with the second reaction zone comprising acetic acid and ethylene with the product streams being subsequently separated thereby producing vinyl acetate. Also, U.S. Pat. No. 6,476,261 describes an oxidation process for the production of alkenes and carboxylic acids such as ethylene and acetic acid, which are reacted to form vinyl acetate, demonstrating that more than one reaction zone can be used to form the vinyl acetate.
This vinyl acetate reaction, however, lends itself to the production of several unwanted impurities, including, for example, non-volatile residues such as polymerized vinyl acetate, polymerized ethylene, and heavy ends, such as acetoxyacetic acid. The formation of these by-products is detrimental in many respects. For example, the formation of these by-products reduces vinyl acetate yield and may lead to fouling of vinyl acetate production equipment, e.g., purification towers and vaporizers.
Conventionally, a heavy ends tower is utilized to remove these by-products. The heavy ends tower, however, is expensive. Also, the level of purification achieved by the heavy ends tower leaves much to be desired. Thus, the need exists for methods for producing a vinyl acetate composition wherein the formation of impurities is inhibited or wherein the impurity content is reduced.
The disclosures of all of the references mentioned above are hereby incorporated by reference.